A former Hong Kong Observatory chief has slammed the government’s heat stress warning system for issuing only the lowest alert during Sunday’s record-breaking heat.

Lam Chiu-ying, who led the Hong Kong Observatory from 2003 to 2009, said it was “truly baffling” that temperatures over the weekend only activated the “Amber” Heat Stress At Work Warning.

He noted reports saying that the amber heat stress warning had been triggered more than 200 times since the system was introduced in May 2023, while the red and black warnings have never been activated.

“Isn’t that a bit strange?” Lam asked in a Chinese Facebook post.

Temperatures reached the highest level of record on Sunday as a heatwave influenced by Tropical Cyclone Dolphin continued to affect Hong Kong. The mercury reached 39.8 degrees Celsius in Sheung Shui on Sunday, while the official temperature at the Observatory headquarters reached 36.9 degrees Celsius.

The warning system consists of amber, red and black alerts and recommends different rest arrangements for employees working outdoors or without air conditioning. None of the terms are legally binding upon employers.

The system recommends rest arrangements depending on the type of work and the heat warning in force. The red and black warnings trigger work suspensions for workers with heavy physical workloads, though they have never been activated.

“If even record-breaking temperatures fail to trigger [the red warning], is it not just a decorative display piece?” Lam asked.

A reading of 30 on the Hong Kong Heat Index triggers the amber warning, and for the red or black warning to be issued, the index must reach 32 and 34, respectively.

In addition to air temperature, the index also takes into account other factors that determine how hot it actually feels, including humidity, wind speed, and solar radiation. Weightings for those factors in the calculation of the index are determined based on hospital admission figures.

Heat index ‘unsuitable’

Lam said that the Hong Kong Heat Index underestimates the heat stress effects of direct solar radiation, especially for outdoor workers. “The solar radiation weighting within the Hong Kong Heat Index is very low, making it unsuitable for outdoor work environments,” he said.

According to the Observatory, the Natural Wet-Bulb Temperature, which measures how effectively sweat can evaporate and cool the body, accounts for 80 per cent of the index figure, as most heat-related hospitalisations are attributed to impeded sweat evaporation.

The Black Globe Temperature, which measures radiant solar heat, only shows a minor correlation with hospitalisations and only counts for 5 per cent of the heat index formula.

| / | Hong Kong Heat Index weighting | Wet Bulb Globe Temperature weighting | Health factor measured |
|---|---|---|---|
| Natural Wet-Bulb Temperature | 80 per cent | 70 per cent | Humidity and sweat evaporation efficiency |
| Dry-Bulb Air Temperature | 15 per cent | 10 per cent | Ambient air temperature |
| Black Globe Temperature | 5 per cent | 20 per cent | Radiant solar heat |

However, these hospitalisation rates represent general population hospital admissions and are not specifically calibrated for outdoor workers such as construction workers and sanitation workers.

Lam suggested that the government adopt the Wet Bulb Globe Temperature (WBGT), another composite heat reading, as an indicator instead. It uses the same measurements as the Hong Kong index but carries a higher 20 per cent weighting for the Black Globe Temperature.

Stronger protections urged

Fay Siu, chief executive of the Association for the Rights of Industrial Accident Victims, said in a Monday press release that the WBGT had reached the “red” warning level on several days recently, compared to the local index. In some cases, the temperature was only 30 degrees Celsius but the humidity was high enough to trigger the amber warning.

The association recommended a dual-track system whereby the index which triggers a warning first would take precedence.

Siu also said workers needed stronger protections, as those who died due to underlying illnesses such as heart conditions are not considered to have died from heatstroke. According to the Labour Department, there have been 20 to 30 heatstroke-related work injuries annually over the past few years, but no fatalities.

If an injury or death must be solely caused by work, “labelling heatstroke as a work injury would be virtually meaningless, and workers and their families will be utterly unprotected,” Siu said in Chinese.

Last month, a construction worker at a public housing worksite fell unconscious and was certified dead at hospital. According to reports, the amber warning had been in effect since that morning, with temperatures reaching 31 degrees Celsius or above.

A Labour Department spokesperson said that the amber warning was issued under the established mechanism as the heat index stood between 30 and 32 at ten stations on Sunday.

Although the temperature was high on Sunday, the relative humidity was below the summer average and wind conditions were good, they said.

The department added it will monitor the weather and the heat stress warning to determine if there is any need for optimisation.

The Intergovernmental Panel on Climate Change has warned that the intensity and frequency of heatwaves have continued to increase since the 1950s due to human-caused climate change. The prevalence of greenhouse gases like carbon dioxide – which trap heat in the atmosphere – raises the planet’s surface temperature, with hotter, longer heatwaves putting lives at risk.

See also: How extreme heat became the deadliest silent killer among world weather disasters

Hong Kong has already warmed by 1.7 degrees Celsius since the Industrial Revolution, research NGO Berkeley Earth says. Heat and humidity may reach lethal levels for protracted periods by the end of the century, according to a 2023 study, making it impossible to stay outdoors in some parts of the world.